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November 23, 1998
By Tim Stephens
A massive plume of hot rock rising through the Earth and erupting through the ocean floor has been building islands in the central Pacific, including those of Hawaii, for at least 80 million years. Now UCSC researchers may have located the origin of the Hawaiian plume at the boundary between Earth's mantle layer and its metallic core.
How deep the Hawaiian plume extends has long been a subject of debate among earth scientists, said Sara Russell, a UCSC earth sciences graduate student. Russell worked with Thorne Lay, professor and chair of earth sciences, and Edward Garnero, research seismologist at the UC Berkeley Seismological Laboratory, on the new study, published in the November 19 issue of the scientific journal Nature.
The researchers found evidence of the Hawaiian plume at the very base of the mantle, about 1,800 miles (2,890 km) beneath the surface of the Earth. Here, the molten outer layer of the Earth's core heats the overlying rock at the base of the mantle.
Using seismic waves generated by earthquakes to probe these deep layers of the Earth, Russell identified a structural pattern in the boundary layer between the mantle and the core that suggests material is flowing horizontally toward the base of the Hawaiian hotspot and then rising vertically.
"We're seeing a change from horizontal to vertical structure that seems to be related to the Hawaiian plume," Russell said.
The Hawaiian hotspot is the most productive plume-related hotspot in the world, said Lay. Its volcanic eruptions have built underwater mountains that eventually emerged from the sea as islands. Hawaii's Mauna Loa, for example, is the most massive mountain on Earth, occupying 10,000 cubic miles and rising 30,000 feet from the seafloor.
Tectonic motion carries the Pacific plate of the Earth's crust over the hotspot, resulting in a series of volcanoes on the seafloor. Extending northwest from Hawaii is a long chain of underwater mountains that once sat above the hotspot, as Hawaii does now, said Lay.
To study the structure of the core-mantle boundary beneath the Hawaiian hotspot, Russell analyzed seismic waves from earthquakes in the region of Tonga and Fiji. The earthquakes, with magnitudes between 5.0 and 7.0, occurred deep in the Earth and generated seismic waves that passed through the deep mantle beneath the Hawaiian hotspot before being detected by seismic instruments on the West Coast of the United States.
"The signals from events of this size are detectable worldwide using sensitive seismometers," said Lay. "While the strongest of these earthquakes are felt in the Tonga and Fiji islands, most are not newsmakers," he noted.
Most of the data used in the study were obtained from seismology recording stations in California and Oregon, said Russell. In recent years seismologists have greatly increased the number of such stations, enabling Russell to conduct a more detailed analysis of deep structure beneath Hawaii than in any previous study.
The basic technique, called seismic tomography, uses the same types of computations that generate computed tomography (CT) scans of the body from x-rays passing through at different angles, said Lay. As seismic waves radiate outward from the epicenter of an earthquake, their speed and other properties are affected by the different types of rock they encounter. By comparing seismic waves that take different paths through the Earth, scientists can draw inferences about the Earth's internal structure.
Russell's study went beyond conventional tomography, which focuses on the travel time of seismic waves, to include analysis of how the seismic vibrations are polarized. This polarization analysis is sensitive to flow-induced structural changes and revealed, for the first time, variations at the base of the mantle suggesting a localized transition from horizontal to vertical flow.
A recently published study by researchers at Harvard University provides unexpected theoretical support for Russell's findings. Published in the Geophysical Journal International, the study predicts where the base of the Hawaiian plume would be if it extends into the lower mantle layer. The Hawaiian plume does not rise straight up through the Earth because it passes through layers of rock in the mantle that are moving horizontally. Based on the expected deflection of the plume as it rises through the mantle, the Harvard researchers predicted its location in the lower mantle to be in the precise area where Russell detected evidence of the transition from horizontal to vertical flow in the core-mantle boundary layer.
Nevertheless, Lay said, additional research is needed to verify that the newly detected structural features are in fact related to the Hawaiian plume. "This will certainly be a provocative paper, and if it turns out to be right it will be a landmark," he said.